Gallagher was born in England but moved to New Zealand as a teenager and went on to star for the All Blacks at international level. He joined Wellington and found his way into the team at centre and swiftly moved into the fullback berth. Having made his All Blacks debut on tour in France, Gallagher was selected in the squad for the 1987 Rugby World Cup.

Gallagher scored four tries in the pool game against Fiji and also scored at try in the quarter-final defeat of Scotland as the All Blacks went on to lift the trophy. Over the next three seasons Gallagher took his international caps up to 18, including 13 tries as part of a formidable New Zealand team.

But in 1990 he returned to the UK and switched codes to take up a contract with Leeds rugby league club. His transition to league, after making 88 appearances for Wellington, was not as successful as promised and he returned to union a few years later. Though, he never rose back to international level.